Generally, electromechanical components are used in single-modulator sequential-color display systems to produce full-color projected images. In such systems, a color wheel is typically spun at high rotations per minute to minimize color break-up artifacts and support a 480 Hz field rate. The duty cycle for such devices is determined by the “spoke-time.” The “spoke-time” is the time required for a spot to transition from one color segment to the next color segment. Typically, color wheels include two color segments for each primary color (red, blue and green). The relative field duration of color wheels is determined by the angular extent of each color segment. Color wheels provide high throughput and are polarization independent. Thus, color wheels are suitable for polarization independent applications such as digital micromirror devices.
Typically, black wedges are introduced between color segments on a color wheel to provide blanking for a display during loading/settling of an image. The blanking reduces projector efficiency for meeting color gamut requirements. Further, the mechanical rotation of color wheels poses difficulties in high-vibration environments such as, applications developed for commercial or military aircraft, automobiles, or other mobile applications or applications otherwise subject to vibration or acceleration. More recently, color switches are being used as an alternative for color wheels in projection displays. Typically, liquid-crystal-based color switches are used with various color-selective components such as, dichroic mirrors, pleochroic dye color polarizers, cholesteric liquid crystals, and retarder stacks. Some color modulators include liquid-crystal polarization switches. Typically, these color modulators are used in CRT-based displays to obtain shadow-mask-free, high-resolution color displays.
Some reflection-mode projection display systems include retarder-stack-based color switches. The retarder-stack-based color switches are used to modulate the polarization of colored light onto a display panel by placing a retarder stack between a Polarizing Beam Splitter (“PBS”) and a Liquid Crystal on Silicon (“LCoS”) display panel. Retarder-stack-based color switches with a single stack and a single reflective modulator are also used for white/primary switching similar to transmissive mode projection display systems.
Retarder-stack-based color switches provide full-color in transmission mode. These color switches include separate red, green, and blue analyzer stages, each independently operating on the polarization of one primary color. The separate analyzer stages permit additive mode switching, which optimizes the chrominance of the additive primary outputs and the black state. However, the manufacturability of a full-color retarder-stack-based color switch is substantially difficult because the complexity of a filter stage for the retarder-stack-based color switch increases with wavelength due to inverse wavelength dependence of retardation and birefringence dispersion. This is exacerbated by the characteristic power spectra of ultra-high-pressure mercury (UHP) lamps, which are yellow rich and red deficient. For example, a red filter with a steep transition slope and low cyan (blue+green+yellow) leakage requires a pair of stacks with many layers. Similarly, a blue stage of the retarder-stack-based color switch may contain a total of 8–10 retarder layers, while a red stage may contain 30–36 layers to achieve an acceptable color transition slope.
Other problems with retarder-stack-based color switches include restricted field-of-view, high switching time, and poor throughput. The field-of-view of these color switches is often restricted by the stability of the red color coordinate with incidence angle and azimuth often due to blue/green leakage. This leakage occurs in part because of the large positive z-retardation of the energized pi-cells in the blue/green stage. To overcome incidence angle effects, additional layers are frequently required for compensation. These layers are in addition to compensation required to nullify the residual in-plane retardation of a fully energized pi-cell.
The switching time issues of retarder-stack-based color switches relate to the retardation swing necessary to fully modulate the red field. For example, the modulation limit can be reached in most commercially available liquid crystal fluids in an in-plane compensated pi-cell for 310 nm of on-state retardation. In elevated temperature conditions, the 0–100% time constant of these color switches can exceed 1 ms, and in some instances, 100% transmission is never reached.
Because these color switches use a common-path, each color must pass through all three stages. Thus, it is difficult for a color switch to match the insertion loss of a color wheel. The manufacturing tolerance of these color switches can influence red saturation, and a delicate balance exists between stacks on either side of a pi-cell. Any error in this balance can cause leakages when the pi-cell is driven high. Such leakages are often cumulative due to the interaction between various stages on a field basis. Thus, manufacturing issues pertain largely to the lack of independence in manufacturing retarder stacks. In practice, it is challenging to assess the spectral leakage characteristics of a particular stack until it is paired with its counterpart. Conversely, forcing a stack to perform well relative to an arbitrary standard is overly restrictive, which unnecessarily reduces yields. Conventional transmissive color-switches also have parallax issues due to the longitudinal path-length between patterned devices. There are currently no practical methods for implementing this without bulkiness or high cost.